Motion control refers to electromechanical systems which produce a desired motion in a mechanical load in response to a planned motion path. Such servo systems improve performance and productivity in automatic machinery used by manufacturing, testing, vibration control and other industries. In order to provide accurate motion control for these systems, it is necessary to accurately measure the position of the load.
FIG. 1 shows a typical prior art servo system 10 consisting of a motor 12, its moving load 14, a position sensor 16, including a stationary sensor head having two analog sensors thereon, a moving motor shaft 18 and an interpolator 20 which provides a digital signal and a servo controller 22 which uses an external digital signal and the digital signal from the interpolator 20 to provide a control signal to the motor 12 to achieve controlled linear motion. This invention can also be used to control rotary motion. The moving motor shaft 18 provides a field related to the magnet pole pair spacing thereof to be sensed, which varies along the length thereof. A measurement of the field detected by the sensors 16 can be correlated with the position of the motor shaft 18 relative to the stationary position of the motor. The variation can be sinusoidal along the length thereof or can vary in some other manner.
In one prior art system, two sensors 24 on the sensor head are located to detect a sinusoidally varying moving shaft field 25, the respective positions of the two sensor heads 24 being separated by a distance equal to 90 degrees of the sinusoidal signal period so that there is a sine signal and a cosine signal. Since the moving load 14 is driven by the moving motor shaft 18 the positions thereof are in a fixed relationship. The interpolator 20 converts easily detectable relatively coarse positioning data from the two sensors 24. The analog portions of the data from the two sensors are converted into a higher resolution signal for use by the motion control system 10 by manipulation thereof.
Typically the control signal is supplied to the system as electrical, but it can also take the form of pneumatic, hydraulic, or other power sources. The sensors can be of a type appropriate for the field being sensed, whether HALL or GMR devices for sensing magnetic flux density, or optical devices for sensing light, or other appropriate means of sensing a periodically varying field with respect to position.
It is well-known to use an arrangement of two position sensor elements as in FIG. 1 to directly output sensed signals in one of two commonly used incremental formats pictured in FIGS. 2A and 2B as for two periodic sensor signals, Analog A Quad B format 30 or in Digital A Quad B format 32.
The term incremental means that the position signal is provided in the form of electrical signals which can be used to provide a positional count by increments. The encoder signal supplied as two sine signals offset by 90 degrees (to comprise a sine/cosine pair) is commonly referred to as being in the Sin/Cos or Analog A Quad B format. The encoder signal supplied as two digital square wave signals, likewise offset in phase by 90 degrees is commonly referred to as being in the Digital A Quad B format.
If a position sensor array provides signals in either the Analog A Quad B format or in the Digital A Quad B format, a digital counter can keep track of a position measurement by counting the number of zero crossings of the sine and cosine signals over time. Since only four zero crossings are provided per electrical cycle, the zero crossing count resolution is coarse. As an example, this would provide a position measurement resolution of one quarter inch for those systems with a magnet pole pair spacing of one inch.
A common position resolution requirement in motion control is less than 1/2000th of an inch, so it is desirable to achieve a much finer position resolution than that directly output by sensing the zero crossings of the pair of sensed analog field signals. Finer resolution is provided by an interpolator.
A prior art position interpolator, such as the interpolator 20 of FIG. 1, senses a sensor signal pair in the Analog A Quad B format 34 measured at a relatively coarse resolution and converts it into a Digital A Quad B format 36 at a higher equivalent resolution as described below and as illustrated in FIGS. 3A and 3B.
The motion control system 10 can receive this Digital A Quad B formatted signal 36 which is compared to signals representing the desired path 38 to provide servo positioning signals to drive the servo motor 12 with a greater degree of precision than possible without the interpolator 20. A Digital A Quad B formatted signal can also be used by the motor drive as commutation data for certain types of motors to generate drive signals.
Prior Art Interpolators
Interpolation is the process of subdividing the quarter cycle intervals into fractions thereof. Referring now to a prior art system 40 of FIG. 4 for identifying a position of a motor shaft 42 along a path 44, the shaft 42 and sensor 46 in combination convert the shaft position into a pair of signals defined by the shaft magnetic field 50, which in the prior art sinusoidal interpolator vary trigonometrically along the shaft.
The sinusoidal interpolator 54, commonly known as a “Sinusoidal Vector Follower,” functions as follows. A pair of sensed sine/cosine signals 48 is combined using mathematical operations with a locally generated trigonometric signal 56 from signal generator module 60 to produce a single error signal 58. This error signal 58 represents a comparison between the common phase angle used to produce the sine and cosine signals 48 and a phase angle used to produce the locally generated trigonometric signal.
If the sine and cosine signals are matched, the common phase angle used to produce the sine and cosine signals is linearly related to the position of the shaft field and therefore is a linear function of the shaft position. If the sine and cosine signals are distorted instead of matched, the common phase angle used to produce the sine and cosine signals is non-linearly related to the position of the shaft field and therefore is a non-linear function of the shaft position.
The phase angle used to produce the locally generated trigonometric signal is determined by the interpolator circuitry to be a linear function of the interpolator output position. This may be accomplished as in FIG. 4 by converting the interpolator output position signal 62 to an index signal 66 to address a lookup table 62 containing a tangent lookup function. Lookup table 62 may be stored in memory, for example, in signal generator module 60.
In order to interpolate changes in shaft position which generate changes in phase angle greater than 360 degrees of phase, the index signal 66 can be generated as a modulus, or repeating function, so that the index signal addressing the table 62 can, upon reaching an address at the end of the table, wrap around to the beginning of the table 62.
As such, the error signal 58 indicates the direction in which to increment or decrement the locally generated angle or position signal to more closely approach the measured angle or position signal.
The position output can be formatted by appropriate operations for output to the motion system as a parallel digital word, or the position output may be converted to another format such as Digital A Quad B.
Common Sensor Signal Deviations
Sensor signals usually differ from ideal predicted values, and are often not necessarily trigonometric. If sensor signals were perfect, prior art system 40 described above could provide a completely linear output representation of the shaft position. However, the following variations in sensor signals commonly occur, some of which the prior art system cannot compensate for.
First of all, a distinction must be made between predicted variations and unpredicted variations. Unpredicted variations are those in which sensor signal variation from the ideal is not predicted, such as temperature and sensor distance from shaft, and for all other varying conditions, including sensor manufacture, which affect the manner in which the position sensor system operates. Such variations could be made predicted variations by performing scaling and offset operations on the sensor signals in response to measured conditions, or by calibrated test runs during sensor manufacture. For example, a difference in sensor offset voltage will occur in most Hall sensors at zero sensed field as a natural consequence of manufacturing tolerances. This offset voltage can be measured prior to sensor installation and supplied to the sensor system as an offset correction voltage as part of the normal interpolator operation.
Sensor signal wave shape can predictably vary due to sensor saturation, magnetic field distortion or other factors. For the example interpolator in FIG. 4, a lookup table or other such means can compensate for most predictable or predicted deviations of sensor signal wave shape from that of an ideal sine wave. However, unpredictable or unpredicted variations can not be anticipated and therefore require that the sensor compensate for them during interpolator operation.
Some parameters change as a function of shaft position from one cycle to the next, and can largely be predicted by “mapping” the deviations caused by shaft fabrication tolerances as a function of shaft position. The use by the interpolator of such “mapped” signal deviations is limited to those situations in which the interpolator circuitry can sense which positional shaft cycle is being measured at any given time.
As an example, if it can be predicted that one shaft magnet pole is shaped such that the sensor signal contains 1% total harmonic distortion and that the adjacent shaft magnet pole is shaped such that the sensor signal at that position contains 2% total harmonic distortion, then a lookup table could compensate for the output signal nonlinearities resulting from 1% or 2% total harmonic distortion respectively to achieve an accurate position output signal.
Such predictability disappears, however, if the shaft is moved while operating power is removed from the interpolator. The prior art interpolator as described here has no means of tracking the number of shaft cycles traversed by the interpolator while the interpolator is not operating. Accordingly, those parameters which change from one positional cycle to the next are also referred to herein as being unpredicted.
Unpredicted sensor signal deviations can include a) common amplitude, b) differential amplitude, c) offset voltage, d) electrical interference and noise (EMI), e) sensor signal nonlinearity due to changes in amplitude and temperature and f) changes in cycle pitch.
Methods for compensating changes in common amplitude are known in the prior art. Such changes can occur when the sine and cosine inputs of the interpolator deviate in amplitude by the same relative amount. Such deviations may occur due to unpredicted changes in distance of sensor head from the shaft (“ride height”) and with shaft magnet temperature. Common amplitude changes are typically compensated for by the interpolator due to the ratiometric nature of the design, which compares a sine signal with its cosine counterpart multiplied by a tangent signal. Since a tangent signal is identical to the ratio of sine to cosine, the measurement is inherently ratiometric and therefore insensitive to such common amplitude deviations.
Limitations of the State of the Art Interpolators
It is desirable to provide a sensor capable of interpolating high resolution position with acceptable linearity using as inputs the typical sensor signals having substantially unpredicted deviations from a sinusoid due to the magnetic fields actually found in close physical proximity to motor shafts. Prior art sinusoidal interpolators suffer a number of disadvantages in their design which render them incapable of producing linear position output signals in the presence of such and other sensor signal deviations. Two examples of these disadvantages are: a) dependency upon one sensor signal; and b) sensor signal distortion.
As described above in reference to FIGS. 2A-3B, prior art sinusoidal interpolators are designed so that there exist at least four positions within the interpolation cycle in which the position output signal depends upon one sensor signal alone. This results in a change in the position output signal in proportion to the deviation in a sensor signal at quarter cycle intervals. In particular, referring to FIGS. 4 and 5, an error signal calculator 68 of prior art interpolator 54 produces error signal 58 from the sum of a sine sensor signal measurement and the product of a cosine signal multiplied by a modeled trigonometric (tangent) signal 56.
At a shaft position corresponding to 90 and 270 degrees of phase, the tangent signal becomes very large. Theoretically, the tangent signal becomes infinite, but in practice, the phase angle results in a tangent value of over 100. As a consequence, the cosine sensor signal becomes scaled by a much higher factor than the sine sensor signal. Following this reasoning, the prior art interpolator largely ignores the sine sensor signal at shaft positions corresponding to 90 and 270 degrees of phase.
Conversely, at a shaft position corresponding to 0 and 180 degrees of phase, the tangent signal becomes very small. Theoretically, the tangent signal becomes zero, but in practice, the phase angle results in a tangent value of much less than one. As a consequence, the cosine sensor signal becomes scaled by a much lower factor than the sine sensor signal. Following this reasoning, the prior art interpolator largely ignores the cosine sensor signal at shaft positions corresponding to 0 and 180 degrees of phase. Accordingly, because the prior art interpolator error calculation depends upon one sensor only at four positions, its position output signal is subject to errors which are directly proportional to sensor signal distortion and noise near these positions.
Position signal output nonlinearities are a second limitation of prior art interpolations. Such nonlinearities can be caused by deviations in differential signal amplitude. When one sensor on the leading edge of the array enters a shaft field region where the magnetic field, due to tolerances in magnet manufacture, is of different amplitude, the one sensor may exhibit an unpredicted change in signal amplitude relative to that of the trailing sensor which remains in the original shaft region. A position output signal error will result which is proportional to the differential signal amplitude error near those shaft positions of 0, 90, 180 and 270 degrees.
A position output signal error will also result from unpredicted offset voltage output which can occur at shaft positions where a zero sensed magnetic field is normally expected. This will result when the normally periodic shaft magnetic field is influenced by slowly varying interferences such as fringing fields sensed near the ends of the shaft. Accordingly, the position output signal error will be proportional to the offset voltage error near those shaft positions of 0, 90, 180 and 270 degrees.
Position output signal noise will also result from electromagnetic interference (EMI). EMI can be unpredicted or chaotic. As a result, the position error will be proportional to the amplitude of the EMI near shaft positions of 0, 90, 180 and 270 degrees.
Unpredicted sensor signal nonlinearities can also result in position output signal errors. Signal nonlinearities occur, among other reasons, when varying magnet strength changes the field amplitude into a region of sensor output saturation. The resulting non sinusoidal signal will then fail to match the trigonometric model and the output position signal will become nonlinear.
Fabrication tolerances during shaft manufacture can result in output signal nonlinearities. Sensed signals can vary in phase with respect to one another relative to the resulting variable distance between magnet pole pairs on the shaft. An unexpectedly large shaft pitch will cause the sensors to be less than 90 degrees out of phase, whereas an unexpectedly small shaft pitch will cause the sensors to be greater than 90 degrees out of phase. The resulting signal will then fail to match the trigonometric model and the output position signal will become nonlinear.
Because prior art interpolators are vulnerable to unpredicted variations in sensor signals, and because motion control applications require high levels of accuracy, it is desirable to provide an improved interpolator which can provide a highly linear output position signal in the presence of substantial sensor signal distortion.